If you want to make more you need to scale and scaling probably means starting an agency and turn from programmer to manager/director.
Another approach is to charge more. If you have many clients, it is possible that some of them are willing to pay more than others.
Finally, granted that I don't know anything about your situation, stop and think: do you really want to do this? I mean, 120k/year in most European cities is a very good income.
The more devs you have, the more clients you need, or bigger ones.
So, OP, to be clear, software agencies are lifestyle businesses. Don't expect to scale if you're working on client solutions, because that's what agencies' bread and butter is.
There is a fundamental difference, however, between being a marketing agency, a mobile development shop, or a software company going through RFP cycles for govt. contracts and being a product or service company.
They feel different, too. There might be similar headcount, but differences in goals are just significant enough to matter.
The latter actually scales, but can still be a lifestyle company, too.